Aluminum hydroxide is produced on an industrial scale by well-established methods, such as the Bayer Process. The process operators optimize their methods so as to produce the greatest possible yield from the aluminate process liquors while trying to achieve a given crystal size distribution of aluminum hydroxide product. It is desirable in most instances to obtain the product of relatively large crystal size since this is beneficial in subsequent processing steps required to produce aluminum metal. Production is often limited by processing conditions under which the crystallization and precipitation is conducted. These processing conditions vary from one plant to the next and include, but are not limited to, temperature profiles, seed charge, seed crystal surface area, purge of carbon dioxide or flue gases, liquor loading, liquor purity, and the like.
Extensive efforts have been invested into finding chemical additives and methods to achieve the optimal economic recovery. For example, U.S. Pat. No. 4,737,352 (hereinafter the '352 patent) assigned to Nalco discloses a method providing a reduced percent of small size crystals and an increase in the yield of coarser aluminum hydroxide crystals by adding surfactants dissolved in oil to the pregnant liquor during the precipitation phase of the process.
The limitations of yield and particle size of alumina recovered from Bayer process liquors are also disclosed in U.S. Pat. No. 6,168,767 (hereinafter the '767 patent) entitled “Production of Alumina” assigned to Ciba Specialty Chemicals Water Treatments Limited. A water-soluble crystal growth modifier formulation is disclosed comprising a first composition of a polyalkoxylated non-ionic surfactant and a second composition comprising a surfactant, or a precursor thereof, which is not polyalkoxylated. Ethylene oxide (EO) units are identified as essential components of the formulation in the polyalkoxylated non-ionic surfactant, preferably, ethylene oxide and propylene oxide (PO) units which form an ethylene oxide-propylene oxide block copolymer. The '767 patent discloses a composition which contains “substantially no mineral oil or silicone oil” and emphasizes regularly that the “advantage of the crystallization modifiers . . . is that they do not require the presence of oils.” (e.g., see column 2, lines 21-25; col. 4, lines 25-35; col. 5, lines 21-33). The cost effectiveness of these components and their acceptance when compared to the surfactant/oil blends used in the majority of crystallization modifier formulations in most Bayer processing plants today remains questionable.
Also affecting the particle size and product yield parameters in alumina recovery is the presence of oxalate in the pregnant liquor. Sodium oxalate often crystallizes and precipitates from the liquor over essentially the same temperature profiles as does the desirable aluminum hydroxide product. If left undealt with, oxalate is a contaminate that can act as a seed site resulting in generation of too many small hydroxide crystals, thereby lowering average particle size of the aluminum hydroxide product. In addition, oxalate crystals may adhere to the surfaces of growing aluminum hydroxide and incorporate within the precipitated product. This further leads to the development of excessive amounts of extremely finely divided aluminum hydroxide in the aluminum hydroxide washing and calcination processes that follow. Therefore, effective removal of oxalate from the system is crucial for manufacturing of a high quality aluminum hydroxide product.
Typically, untreated precipitation liquors yield sodium oxalate crystals with needle like morphology. One of the effective ways of removing oxalate is to force it crystallize as spherical agglomerates of such needles also known as “oxalate balls.” Oxalate balls co-precipitate with aluminum hydroxide product and can be removed by screening. The effectiveness of screening is higher when oxalate balls are larger in size. Still, if the balls bear in-grown aluminum hydroxide inclusions, screening may also remove useful aluminum values. Thus, the formation of oxalate balls that are larger and substantially free from incorporated aluminum values is most desirable.
Despite the continuous and ongoing development worldwide, the industry demands for economical resolution of the above-described process needs remain unfulfilled. A method of such resolution suitable for obtaining aluminum hydroxide crystals with increased particle size and yield, while facilitating oxalate removal is provided by the present invention.